1. Field of Invention
The present invention relates to the field of electrical protection. It relates in particular to an automatically quenching surge arrester arrangement as claimed in the preamble of claim 1, and to the use of a surge arrester arrangement such as this.
2. Description of Related Art
Lightening strikes or other brief phenomena can result in dangerous voltage spikes occurring in electrical and electronic circuits, or on electrical cables, which are connected to exposed apparatuses such as radio antennas, and these can lead to permanent damage to the electrical devices or to a total failure. In order to render such voltage spikes safe, surge arresters of various designs and methods of operation have for a very long time been installed at suitable points on the device to be protected, which are non-conductive in normal circumstances but are switched through when dangerous overvoltages occur, equalizing out the potential differences that occur.
One possible type of surge arrester is formed by voltage-dependent resistors, for example metal-oxide varistors (MOVs), which are connected between two conductors between which dangerous voltage spikes can occur. At normal operating voltages, the resistance of the varistors is high, so that only a small leakage current flows between the two conductors. When high voltage spikes occur, the resistance of the varistors decreases drastically, so that the desired equalization current can flow. However, problems result in the case of varistors when greatly increased leakage currents flow in them even in normal conditions as a result of internal changes, which load the circuit to be protected and can lead to changes in their operation. It has therefore already been proposed for a switching device which can be activated thermally to be connected in series with a varistor, which interrupts the current through the varistor as the varistor is heated as a result of a major leakage current, and, as a substitute for this, introduces a spark gap as additional overvoltage protection into the disconnected connection (U.S. Pat. No. 4,288,833). The switching device which can be activated thermally is provided by an elastic switching arm which is soldered by one end to the varistor such that it is mechanically prestressed, and which produces an electrical connection to the varistor. When the varistor is heated severely as a result of excessive leakage currents, the solder of the soldered joint melts, and the electrical switching arm is lifted off the varistor by its prestressing, interrupting the current flow through the varistor. As the switching arm is lifted off, a spark gap comes into effect, which is formed between the varistor and the raised switching arm or conductor tips which are provided specifically for this purpose. This surge arrester arrangement has the disadvantage that the change in the arrangement when the switching device is triggered is irreversible. The switching process of the surge arrester which is provided with an external short-circuit device, from the document DE-A1-197 31 312 is also irreversible, in the same way. If the surge arrester is heated excessively, two insulating spacers melt, so that an elastically prestressed short-circuiting bracket can short the two or three connecting contacts of the surge arrester and can thus take over the current flow through the surge arrester.
Another type of surge arrester is formed by gas capsule arresters, in which an overvoltage results in a gas discharge being triggered in a closed gas-filled capsule with two or three electrodes. These arresters are subject to the problem that, once a gas discharge has been triggered, it is maintained at a comparatively low burning voltage. If, for example, a supply voltage for remote electronics which is greater than or equal to the burning voltage, or a high RF power, is applied during normal operation to the gas capsule arrester which is installed in a circuit or cable, the gas discharge continues to burn after the occurrence of a triggering overvoltage spike, and loads the circuit or cable. Additional irreversible switching devices have also already been proposed for gas capsule arresters, which react to excessive heat being created in the arrester and then permanently interrupt the current supply to the arrester (U.S. Pat. No. 4,051,546) or permanently short the arrester by means of a bypass (U.S. Pat. Nos. 3,755,715 or 4,132,915). The thermally activated switching devices can be integrated in the surge arrester (see the abovementioned documents), although they could also be formed separately and can be thermally coupled to the surge arrester from the outside (U.S. Pat. No. 4,275,432). Thermally activated, irreversible shorting devices are also known in conjunction with gas capsule arresters for coaxial cables (U.S. Pat. No. 5,724,220, FIGS. 24 and 25).
A reversible switching device for interruption of the discharge current through a gas capsule arrester is known from U.S. Pat. No. 4,068,277. In this case, a separate contactor is provided, which is equipped with a bimetallic element, operates thermally and whose heating element is connected in series with the arrester. If a discharge, once it has been triggered, remains in existence for a certain time in the arrester, the contactor trips and interrupts the current through the arrester and one line. Once the contactor has cooled down again sufficiently after a relatively long time of, for example, 20-30 seconds, it automatically reconnects the current through the arrester and the one line, thus restoring the initial state. This solution has the disadvantage that a compact and space-saving arrangement is not possible, because of the separate contactor. Furthermore, interruption in one line is unsuitable for applications in which a supply current for other circuit parts is passed via the lines.
Surge arrester arrangements which are integrated in a coaxial conductor arrangement for radio frequencies and therefore have to be suitable not only for very high frequencies but should furthermore also be physically compact, functionally reliable, robust and requiring little maintenance, are subject to particular requirements.
Examples of coaxial conductor arrangements with an integrated surge arrester arrangement but without any additional switching device are disclosed in CH-A5-660 261, EP-A1-0 855 756, or EP-A1-0 938 166 from the same applicant. In order to allow the gas capsule arresters to be reliably switched to the non-conductive state even when a DC voltage or a radio-frequency signal is applied, in the case of a defect with surge arrester arrangements such as these, WO-A1-2004/032276 from the same applicant proposes an additional switching arrangement, which comprises an inductance, an electromagnetically operated interrupter switch, and a diode. This switching arrangement interacts with a series circuit comprising two identical gas capsule arresters. The configuration and method of operation of the arrangement can be found in the cited document.
The switching arrangement which is known from WO-A1-2004/032276 protects the gas capsule arrester reliably against continuous loading, and is automatically reconnected after quenching of the gas discharge in the gas capsule arresters. This arrangement has been proven in practice and can be integrated in coaxial conductor arrangements provided that they are designed from the start for this purpose.
However, there is an urgent need to have an automatically quenching surge arrester arrangement which can be used in particular over a broad bandwidth, is of simple design and can be produced at low cost, but which can also be retrofitted to existing coaxial conductor arrangements with an integrated surge arrester, as described in CH-A5-660 261, without having to make any physical modifications to the coaxial conductor arrangement itself.